Fluidized beds have gained significant importance in the field of gas-solid contacting. They are being used for a wide variety of uses. For instance, in the chemical vapor deposition of a protective coating on particles of phosphor as disclosed in U.S. Pat. No 4,585,673 to Sigai, it is desirable to control the powder movement and temperature within the fluidized bed. U.S. Pat. No. 4,979,830 to Munn discloses a method for adjustably controlling the rate of circulation and temperature gradient in a fluidized bed. U.S. Pat. No. 4,990,371 to Dutta et al. describes a process for fluidizing small particulate solids and at least partially enveloping the solids with a coating material in a cross-current multi-stage fluid bed reactor.
Fluid bed processes are operated either in a batch or continuous fashion. In a batch process, the entire product remains in the reactor or vessel for withdrawal at the end of the run. In a continuous process, the inventory of material in the reactor is usually a small fraction of the through-put and the withdrawal of solids is continuous.
In batch type processes, the reactor is often inverted to dump the powder. While this may be possible in a laboratory sized fluid bed, this is an impractical and dangerous operation for a pilot or production unit. As a result, a smooth walled discharge tube is often attached to the distributor plate at the bottom of the reactor for withdrawing processed powder by gravity at the end of a batch run. This tube is typically closed by a valve or a plug during the run, and is opened for product removal at the end of the run.
Disadvantages are attendant with the use of a smooth walled discharge tube. The powder in the tube is not fluidized during the fluidization of the bed. At the end of the run, the unfluidized powder is mixed in with the fluidized powder. The tube, which blocks the upward flow of gas, forms a stagnant zone which does not undergo the same fluidization conditions as the powder in the reactor. These conditions can cause product nonuniformity at the end of the run.
When cohesive powders, such as lamp phosphors, are utilized, the problems attendant with smooth tubes are amplified. Bridging of the powder particles frequently results in a slow discharge or even the stoppage of powder flow. This problem may sometimes be overcome by increasing the diameter of the tube. The stagnant zone in the fluid bed, however, is enlarged: no fluidizing gas enters the region of the reactor immediately above the tube. Additionally, the amount of unfluidized powder in the tube dramatically increases with the size of the tube since the volume of material in the tube increases as the square of the tube diameter.
In continuous type fluid bed processes, the withdrawal of cohesive powders may be achieved by pneumatic conveying via a flexible hose located in or above the fluidized bed. The suction necessary for the powder removal may be provided either by a blower or by an eductor. However, with this solution to the powder withdrawal problem, difficulties are encountered. Static electricity build up on the hose may reduce powder flow. Since pneumatic systems for fine particles need a gas-solid filtration system at the downstream end of the hose, additional capital expenditure is necessary to achieve high powder capture efficiencies.
Continuous type processes may also use overflow weirs on the walls of the reactor. While weirs are suited for product transfer across the top of fluid beds, unlike the previous three methods, they cannot be used for draining the inventory of a fluid bed.
Heretofore, difficulties have been encountered in maintaining desired flow of particles during the fluidization process and withdrawal of particles from the bed.